BASH: while read without echo + condition = strange behavior

Question

I have a problem with "if statement" inside while read loop.\\n

Example 1:

81 function processconfig2()
82 {
83     pattern='^monthly'
84     while read line
85     do
86         if [[ $line =~ $pattern ]]; then
87             echo $line
88         fi
89     done < /etc/logrotate.conf | awk '/^\/var\/log\/wtmp/, /^}/'
90     
91     exit 0
92 }

Output is empty.\\n After adding after line 89 echo $line:\\n

81 function processconfig2()  
82 {
83     pattern='^monthly'
84     while read line
85     do
86         if [[ $line =~ $pattern ]]; then
87             echo $line
88         fi
89         echo $line
90     done < /etc/logrotate.conf | awk '/^\/var\/log\/wtmp/, /^}/'
91  
92     exit 0
93 }

I've got proper output (condition at line 86 is processed properly):

/var/log/wtmp {
monthly
monthly
create 0664 root utmp
minsize 1M
rotate 1
}

Could someone explain to me this strange behavior, please.

Answer 1

Your problem is that your awk filter is on the output of your read loop. So when you don't echo the whole file you don't output enough for awk to match so it drops all output.

Try without the awk filter to see what I mean.

Alternatively move the awk filter to before the read loop (I imagine this was the idea anyway).

pattern='^monthly'
while read line
do
    if [[ $line =~ $pattern ]]; then
        echo $line
    fi
done < <(awk '/^\/var\/log\/wtmp/, /^}/' /etc/logrotate.conf)

Answer 2

Another working example:

81 function processconfig2()
82 {
83     pattern='^monthly'
84     content=`awk '/^\/var\/log\/wtmp/, /^}/' /etc/logrotate.conf`
85     while read line 
86     do  
87         if [[ $line =~ $pattern ]]; then
88             echo $line"twice"
89         fi
90     done <<< "$content"
91     
92     exit 0
93 }